Activate GitHub Actions CI workflow

## What
`CI_WORKFLOW_DRAFT.yml` is ready but can't be activated because GitHub PAT lacks `workflow` scope.

## Steps
1. Go to github.com/settings/tokens
2. Edit the PAT to include `workflow` scope
3. Rename `.github/CI_WORKFLOW_DRAFT.yml` to `.github/workflows/tests.yml`
4. Push

## Impact
Automated testing on Python 3.9-3.12 for every push/PR.